Gas components of bubbles generated in electrical discharge machining (EDM) were analyzed using gas chromatography to understand the phenomena related to evaporation and dissociation of dielectric liquids caused by discharge. When EDM oil was used as the dielectric, more than 50% of the gas components was found to be hydrogen. The other gases included oxygen, nitrogen, and hydrocarbon gases such as methane, ethylene, and acetylene. With increasing discharge energy, the percentages of hydrogen and hydrocarbon gases with smaller molecular weight increased due to the dissociation of the dielectric molecules. On the other hand, when deionized water was used as the dielectric, which is normally used in wire EDM, bubbles were composed of hydrogen, oxygen, and nitrogen. The ratio of oxygen to hydrogen was smaller than the ratio determined by stoichiometry due to oxidation in the gap. In the presence of equivalent discharge energy, the volume of bubbles generated in wire EDM using deionized water was significantly smaller than that in sinking EDM using oil. This is because hydrogen and oxygen generated due to dissociation in the discharge plasma recombine and condense to water after discharge.
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